resources/xaml/Themes/SharedStyles.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
 
    <Style TargetType="{x:Type ScrollBar}">
        <Setter Property="Background" Value="Transparent"/>
    </Style>
 
    <Style TargetType="{x:Type TabItem}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type TabItem}">
                    <Grid x:Name="Root">
                        <Border x:Name="Border"
                                Background="Transparent"
                                BorderBrush="Transparent"
                                BorderThickness="0,0,0,3"
                                Margin="0,0,4,0"
                                Padding="12,8,12,8">
                            <ContentPresenter x:Name="ContentSite"
                                              VerticalAlignment="Center"
                                              HorizontalAlignment="Center"
                                              ContentSource="Header"
                                              RecognizesAccessKey="True"/>
                        </Border>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsMouseOver" Value="True">
                            <Setter TargetName="Border" Property="Background" Value="{DynamicResource ControlBgBrush}"/>
                            <Setter Property="Foreground" Value="{DynamicResource ControlFgBrush}"/>
                        </Trigger>
                         
                        <Trigger Property="IsSelected" Value="True">
                            <Setter TargetName="Border" Property="BorderBrush" Value="{DynamicResource AccentBrush}"/>
                            <Setter Property="Foreground" Value="{DynamicResource AccentBrush}"/>
                            <Setter TargetName="Border" Property="Background" Value="Transparent"/>
                        </Trigger>
 
                        <Trigger Property="IsSelected" Value="False">
                            <Setter Property="Foreground" Value="{DynamicResource SecondaryTextBrush}"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
 
</ResourceDictionary>